On Mon, Mar 08, 2021 at 09:13:55AM +0000, David Howells wrote: > Amir Goldstein <amir7...@gmail.com> wrote: > > With ->fiemap() you can at least make the distinction between a non existing > > and an UNWRITTEN extent. > > I can't use that for XFS, Ext4 or btrfs, I suspect. Christoph and Dave's > assertion is that the cache can't rely on the backing filesystem's metadata > because these can arbitrarily insert or remove blocks of zeros to bridge or > split extents.
Could you instead make some sort of explicit contract with the filesystem? Maybe you'd flag it at mkfs time and query for it before allowing a filesystem to be used for fscache. You don't need every filesystem to support fscache, right, just one acceptable one? --b.
